Upgrade lib v0 uncaught exception on scale-up after `pre-upgrade-check`

## Steps to reproduce

```
juju deploy mysql-k8s --channel 8.0/stable --trust
juju run mysql-k8s/0 pre-upgrade-check
juju scale-application mysql-k8s 3
```

## Expected behavior

App scales

## Actual behavior

Uncaught exception on `upgrade-relation-changed` lib hook for new units only.

## Log output

```
unit-mysql-k8s-2: 01:35:57 ERROR unit.mysql-k8s/2.juju-log upgrade:5: Uncaught exception while in charm code:
Traceback (most recent call last):
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/./src/charm.py", line 888, in
main(MySQLOperatorCharm)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/main.py", line 551, in main
manager.run()
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/main.py", line 530, in run
self._emit()
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/main.py", line 519, in _emit
_emit_charm_event(self.charm, self.dispatcher.event_name)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/main.py", line 147, in _emit_charm_event
event_to_emit.emit(*args, **kwargs)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/framework.py", line 348, in emit
framework._emit(event)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/framework.py", line 860, in _emit
self._reemit(event_path)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/framework.py", line 950, in _reemit
custom_handler(event)
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/lib/charms/tempo_k8s/v1/charm_tracing.py", line 724, in wrapped_function
return callable(*args, **kwargs) # type: ignore
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/lib/charms/data_platform_libs/v0/upgrade.py", line 987, in on_upgrade_changed
top_state = self.peer_relation.data[top_unit].get("state")
File "/var/lib/juju/agents/unit-mysql-k8s-2/charm/venv/ops/model.py", line 1671, in __getitem__
return self._data[key]
KeyError:
```